Upper limits on radio emission from the K2-18 system

ABSTRACT Stellar and planetary magnetic fields play a crucial role in the habitability of a planet and the integrity of its atmosphere. The detection of methane and carbon dioxide, along with a tentative identification of the potential biosignature dimethyl sulfide/disulfide, in the atmosphere of K2-18 b, a sub-Neptune orbiting an M dwarf star, presents an intriguing question regarding the stellar magnetic environment and the resistance of the planet’s magnetosphere (if it exists) to erosion by magnetic activity from the host. To probe for radio emission from the system, we have conducted observations using the Karl G. Jansky Very Large Array at S, C, and X bands (2–4, 5.5–7.5, and 8–10 GHz, respectively) to search for coherent and incoherent radio emission. We detect no radio emission associated with incoherent emission mechanisms. We report $3\sigma$ Stokes I upper limits of $49.8\ \mu \rm {Jy\,beam}^{-1}$ at S band, $17.7\ \mu \rm {Jy\,beam}^{-1}$ at C band, and $18.0\ \mu \rm {Jy\,beam}^{-1}$ at X band and an upper limit of the ratio of the radio to the total bolometric luminosity of $\log L_\text{R}/\log L_\text{bol}< -8.8$. We have also searched for short duration bursts associated with coherent emission mechanisms at C and X bands. No signals above a $3\sigma$ significance threshold are detected. Although no signals are detected, our radio observations offer constraints, albeit limited, on the stellar magnetic environment supporting recent X-ray observations indicating that K2-18 is a very faint emitter. Our results also contextualize any planetary transmission spectra by providing constraints on the activity level of the host.